Metaphor is the comparison of two concepts through language, often done by using the verb 'to be'.  Metaphor is often seen as a literary device used by poets. Metaphors, however, can be found in every day speech and images.

Take for example this image, which is an advertisement for a car, the Volvo 850. It compares the car to an egg carton. We assume they share properties. They are both vehicles that protect their content securely. In every day speech, people say things like one is 'a beacon of knowledge,' or a 'light at the end of the tunnel.' In essence these are metaphors as well. 

Metaphor should not be confused with simile which is also a linguistic device that compares two things. Simile, however, uses words such as 'like' or 'as' in order to compare two concepts.